A centrifugal separator of this kind is shown in WO 89/03250. During operation of the shown centrifugal separator the liquid present in the outlet chamber is entrained gently into the rotation of the rotor by means of discs, which are arranged in the outlet chamber. The separated liquid is discharged out of the outlet chamber through a stationary discharge device arranged therein, which is connected to an outlet. The flow of the discharged liquid depends on the angular speed of the rotating liquid in the outlet chamber, the radial position of the free liquid surface, the design of the discharge device, and the prevailing counter pressure in the outlet for the liquid.
For every use of such a centrifugal separator a discharge device is chosen having for the actual case a suitable flow capacity range, which is limited for one and the same discharge device. Within the flow capacity range the flow is regulated by adjusting the counter-pressure in the outlet. If the actual flow of the separated liquid is low the counter-pressure from the equipment connected to the outlet often is insufficient and an extra counter-pressure has to be imposed at the outlet, which means energy losses. Besides, a low flow through the discharge device often results in an unstable flow, which in turn gives rise to rotor dynamical oscillations.